Hi. I am not speaking from personal experience but I have heard of woman with a PWS stating that during pregnancy it did appear to get darker etc. The hormone's during pregnancy did seem to cause darkening, increase in blebs etc. in some but not everyone. Some have said that they saw no noticeable change during their pregnancy.
If this is a concern before you decide to become pregnant, ask your doctor. Please make sure that they are familiar with Vascular Malformations....
good luck
Elena